This invention relates generally to liners for shaped charges and more particularly to liners for shaped charges of the type used in perforating gun systems.
Development of an oil or gas well often involves fixing a tubular steel casing in cement in an underground well borehole. Holes, or perforations, are subsequently created in the steel well casing and surrounding cement in order to gain access to the surrounding formation, e.g., an oil or gas bearing stratum. Such holes are generally created through a process known also as perforation using a perforating gun.
Perforation is a process of piercing the well casing, the surrounding cement, and rocks in the surrounding formation to provide fluid communication between the oil or gas deposit and the interior of the well. Explosive charges are typically used to pierce the well casing. Perforation involves introducing a firing device, commonly termed a perforating gun, into the well, positioning the perforating gun at a desired depth, and detonating the gun. The process of locating a perforation gun in position is sometimes referred to as xe2x80x9cdeliveringxe2x80x9d the gun. After detonation, the gun may be retracted or released and dropped to the bottom of the well. If discarded, the size of the gun is limited by the depth of the bottom hole available.
Several perforating methods have been used to deliver and detonate a perforating gun. For example, the xe2x80x9cwirelinexe2x80x9d process involves attaching a perforating gun, or string of guns, to a long, flexible steel cable paid out from a truck-mounted drum at the surface. An electrical conductor, paid out with the cable and connected to the gun, or the gun in the string nearest to the surface, carries an electrical signal to energize the perforating gun detonator. Alternatively, tube conveyed perforating (TCP) employs straight production tubing to carry or convey the perforating gun to a desired position in the well. The gun may be activated by a drop bar. The gun may also be activated by hydraulic means. For example, fluid in the tubing may be pressurized sufficiently to activate a hammer and firing pin of a percussion detonator on the gun.
An explosive charge is typically contained in a charge assembly that may include a casing and a metal liner. The casing may typically have a recess having an inner wall structure and an opening. The explosive charge is packed against the inner wall of the charge casing. The metal liner may line the explosive charge opposite the casting. As such the explosive charge is contained between the liner and the casing. The shape of the explosive charge is defined by the space between the inner wall of the casing and the metal liner and is thus referred to as a xe2x80x9cshaped chargexe2x80x9d. This space often has a concave, typically generally conical, shape. The term xe2x80x9cshaped chargexe2x80x9d may also refer to a charge assembly. The shape of such a charge may be varied, depending on the pattern of perforations desired, the size and number of perforations desired, and the depth of the perforations in the surrounding mineral bearing stratum.
A perforating gun may typically include an elongate member in the nature of a hollow tube having openings, or seats, into which the shaped changes are mounted. Several charge assemblies are generally arranged along the length of the elongate member. Typically, a detonation cord runs along the perforation gun between, and is connected to, the charges. Typically, the shaped changes are mounted such that the wide part of the conical shape faces radially outwardly, i.e., away from the gun, and toward the wall of the well bore, generally having a central axis aimed in a plane transverse to the length of the elongate member. Different explosive charges may face radially outwardly in different angular directions in the plane or in spaced, parallel planes to produce a helical perforation pattern.
When detonated, each shaped charge produces a compressive shock wave. This may collapse the liner and propel the central portion of the liner at a very high speed, possibly of the order of about 10,000 m/sec, thereby forming an explosive central jet. This jet pierces the well casing and the surrounding cement, and penetrates some distance into the oil bearing formation. The differently facing charges explode radially outwardly in different angular directions into the oil-bearing formation. The result is a perforated wall, like a colander, that facilitates entry of oil or gas into the well.
The outer, slower moving portion of the liner may have a tendency to form a slug, sometimes called a xe2x80x9ccarrotxe2x80x9d due to its shape. The slug can cause numerous problems. The slug may embed itself in the exit hole of the perforating gun and cause mechanical difficulties in removing the perforating gun from the well borehole. The slug, when embedded in the perforation pierced by the explosive jet, may tend to impede the outflow of oil or gas, thus reducing the performance of the well. Sometimes, the slug may be carried by the gas or oil flow to the surface and cause malfunction of surface devices. The slug may also fall from the perforation gun down the well borehole into other downhole devices, possibly causing these devices to malfunction.
These problems caused by slugs have long been recognized. Efforts continue to be made to minimize or eliminate the formation of slugs. For example, U.S. Pat. No. 5,098,487, issued to Brauer et al. on Mar. 24, 1992 (xe2x80x9cBrauerxe2x80x9d), gives an account of various solutions directed at minimizing slug formation.
The majority (perhaps up to 90%) of liners used in the field are constructed of compacted metal powders. Metal powder liners tend to pulverize upon striking the well casing, and thus do not tend to cause undue formation of slugs. However, this type of liner may tend to have other disadvantages. They tend to be used in a green (i.e., unsintered) state, and as such may tend also to be relatively fragile. Care must be taken in their handling and assembly. Sintering, such as the process disclosed in U.S. Pat. No. 6,012,392, issued to Norman et al. on Jan. 11, 2000 (xe2x80x9cNormanxe2x80x9d) may reduce this fragility, but is sometimes considered an unnecessary manufacturing step, particularly when it is often desirable for the device to fragment upon detonation.
Compacted metal powder liners in the green state also tend to be porous. There may be water at a depth in the well at which the shaped charge is to perforate holes in the well casing. Water may leak more easily through a porous liner and dampen the explosive charge lined by such a liner. This may cause detonation difficulties.
Often, liners made of compacted metal powders tend to be formed individually. Compared with liners formed in batches, individually formed liners may tend to have increased cost, and their product quality may tend to be less consistent and reliable. Additionally, because liners made of unsintered metal powders often pulverize upon striking the well casing, they may tend to be less effective for perforating large holes.
xe2x80x9cLarge holesxe2x80x9d in this context may be holes of diameter up to about 1 inch. xe2x80x9cLarge holesxe2x80x9d are often required for wells of heavy oil. Heavy oil, having higher viscosity, may tend to flow more easily from the surrounding oil bearing formation through these large holes and into the well. To obtain good production performance from such a heavy oil well, deep penetration with a depth of up to 30 inches may often be desired as well. Solid liners made of relatively heavy material may tend to be more effective for producing holes satisfying those requirements. This type of liner typically accounts for most, if not all, of the remaining 10% of liners in use. However, this type of liner has the tendency of forming relatively large slugs.
Zinc and zinc alloys have been used as a material for the outer casings of shaped charges. A casing made of zinc or a zinc alloy may tend to disintegrate without forming significant debris upon explosion of the explosive charge contained inside such a casing. The long held belief has been that some other material is required for the liner. Efforts have been made in searching for a better liner material. This may involve the development of special alloy mixes of copper.
For example, Brauer (quoted above) discloses a specially chosen copper alloy for making metal liners. The alloy, when heated to a temperature in the liner following detonation, is said to have a ductile matrix with a molten second phase dispersed within the matrix. Brauer states that the molten second phase reduces the tensile strength of the matrix so that the liner slug pulverizes on striking a well casing.
It has been observed by the present inventor that it would be advantageous to employ a liner that is made of a material of which the largest component is zinc. Advantageously, the liner may be made from the same material for making the casing. This material does not tend to require special alloy combinations, tends to be readily available, and tends to be relatively inexpensive. It tends not to require careful mixing of special expensive or exotic copper alloys. A number of methods are suitable for manufacturing zinc-based casings and liners. The present invention is part of the continuing efforts directed at overcoming the foregoing and other attendant difficulties. The liner disclosed herein may tend to be particularly useful in situations where it is required to perforate large holes, with reduced tendency to form slugs, although its application is not limited to such.
In an aspect of the invention there is a shaped charge comprising a casing, a liner, and an explosive charge. The casing has an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth. The liner has a central region and a divergent skirt extending from the central region. The central region lies closer to the apex than to the mouth, and the skirt is mounted to the divergent wall structure closer to the mouth than to the apex. A charge cavity is defined between the casing and the liner. The explosive charge is contained within the charge cavity. The liner is formed from a metal material whose greatest component, by weight, is zinc.
In an additional feature of that aspect of the invention, the metal material of the liner is predominantly zinc. In another additional feature, the metal material of the liner is more than 70% zinc by weight. In yet another additional feature, the metal material of the liner is more than 90% zinc by weight.
In still another additional feature, the metal material of the liner is made of a metal material that is essentially zinc. In a further additional feature, the liner is made from zinc, aluminium and magnesium. In yet a further additional feature, the liner and the casing are made of the same material. In still a further additional feature, the casing is formed from a metal material whose greatest component, by weight, is zinc. In another additional feature, the liner and the casing are made of a metal material that is at least 50% zinc by weight.
In yet another additional feature, the shaped charge has an axis of symmetry, and the casing has the form of a body of revolution relative to the axis of symmetry. In still another additional feature, the charge cavity has a first region lying on the axis of symmetry, and a second region lying radially away from the axis of symmetry. The second region has a diminished thickness relative to the first region. In still yet another additional feature, the liner has a conic region lying at a conic angle relative to the axis. In a further additional feature, the conic angle is in the range of 15 to 40 degrees. In yet a further additional feature, the liner is of constant thickness. In still a further additional feature, the central region of the liner forms a bottom region of a valley and the divergent skirt forms sloped sides of the valley such that the liner is generally V-shaped. In another additional feature, the liner is a casting.
In another aspect of the invention there is a method of manufacturing a shaped charge. The method comprises the steps of providing a casing having an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth, forming a liner from a metal material whose greatest component, by weight, is zinc. The liner has a central region and a divergent skirt extending from the central region, providing an explosive charge and locating the explosive charge in the casing, locating the liner in position relative to the casing to form a charge cavity containing the explosive charge. The step of locating the liner in position includes the step of locating the central region of the liner closer to the apex of the casing than to the mouth and locating the skirt adjacent to the divergent wall structure closer to the mouth than to the apex.
In an additional feature of that aspect of the invention, the step of forming the liner includes the step of providing a material whose greatest component, by weight, is zinc. In another additional feature, the step of forming the liner includes the step of providing a liner material that is predominantly zinc. In yet another additional feature, the step of forming the liner includes the step of providing a liner material that is more than 70% zinc by weight. In still another additional feature, the step of forming the liner includes the step of providing a liner material that is more than 90% zinc by weight. In still yet another additional feature, the step of forming the liner includes the step of providing liner material that is essentially zinc.
In a further additional feature, the step of forming the liner includes the step of making the liner and the casing of the same material. In yet a further additional feature, the step of forming the liner includes the step of making both the liner and the casing of materials that are at least 50% zinc by weight. In still a further additional feature, the step of forming the liner from a powder metal. In another additional feature, the step of locating the liner includes the step of pressing the powder metal liner in a green state against the explosive charge. In yet another additional feature, the step of forming the liner includes sintering the powder metal.
In still another additional feature, the step of forming the liner includes the step of using a metal material of constant thickness. In a further additional feature, the step of forming the liner includes casting the liner. In yet a further additional feature, the step of forming the liner includes machining the liner.
In another aspect of the invention there is a kit for assembling into a shaped charge for receiving and retaining an explosive charge for use with perforating guns, comprising a casing and a liner. The casing has an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth. The liner has a central region and a divergent skirt extending from the central region. The skirt is mountable to the divergent wall structure to form a charge cavity between the liner and the casing for containing the explosive charge. The liner is mountable to the casing, and the skirt is closer to the mouth than to the apex and the central region lies closer to the apex than to the mouth upon mounting the skirt to the divergent wall. The liner is formed from a metal material whose greatest component, by weight, is zinc.
In an additional feature of that aspect of the invention, the metal material of the liner is predominantly zinc. In another additional feature, the metal material of the liner is more than 70% zinc by weight. In yet another additional feature, the metal material of the liner is more than 90% zinc by weight. In still another additional feature, the metal material of the liner is essentially zinc. In still yet another additional feature, the casing is formed from a metal material whose greatest component, by weight, is zinc. In a further additional feature, the liner and the casing are made of the same material. In yet a further additional feature, the liner is a casting.
In another aspect of the invention there is a rail method of manufacturing a shaped charge liner for use with a casing for containing an explosive charge. The casing has an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth, comprising the steps of forming a liner. The liner has a central region and a divergent skirt extending from the central region. The skirt is mountable to the divergent wall structure to form a charge cavity between the liner and the casing for containing the explosive charge. The skirt is closer to the mouth than to the apex and the central region lies closer to the apex than to the mouth upon mounting the skirt to the divergent wall. The liner is formed from a metal material whose greatest component, by weight, is zinc.